The temperature of gas at the outlet from a gas turbine is very high, and at present may be as much as 650.degree. C. The inlet and deflection ducts must therefore be designed to withstand such temperatures. Given their large size, they cannot be made entirely out of stainless steel because of the expansion problems that would result. They are therefore generally made of "warm casing" metal sheet constituted by a sheet of steel, a thermally insulating layer, and tiles of stainless steel organized in the form of scales. Thus, the steel sheet, in particular a sheet of E24 steel, is maintained at a temperature of about 320.degree. C. In addition to being relatively complex to make up, such a sheet also poses problems of being relatively fragile.